1. Field of the Invention
The present invention generally relates to a communication system, and more particularly to a receiving diversity apparatus and method for a mobile station of a mobile communication system.
2. Background of the Related Art
A diversity method is a technology used for improving the quality of a received signal in a wireless communication system. A receiving device in a system applying a diversity method comprises two or more antennas, and respective signals received through the antennas are selected or combined to achieve a received signal quality higher than that of a conventional receiving device which uses one antenna
Generally, there are three types of diversity methods: a selection or switching method, an in-phase combining or equal-gain combining method, and a maximal ratio combining method.
(1) Selection or Switching Method.
The selection or switching diversity method (hereinafter, referred to as selection diversity method) selectively receives the strongest signal among signals received through a plurality of antennas.
(2) In-Phase Combining or Equal-Gain Combining Method.
The in-phase combining diversity method controls phases of signals received through the respective antennas to be in-phase. These signals are then combined to form a final received signal. The in-phase combining diversity method therefore combines the signals, and does not select the signal as in the selection diversity method. As a result, an improved quality of the received signal can be achieved
When implemented, the in-phase combining diversity method requires a phase shifter for controlling phase of the signal. This method is also able to improve the received signal quality under a static environment, where the strength of the received signal is not changed according to a lapse of time or form a fading environment which may change the strength of the received signal according to the lapse of time.
In the in-phase combining diversity method, if one of the received signals has small electric power compared to other received signals only a small amount gain can be added to the signal component in the final combined output. However, because noise is the same as that of the other signals, the signal-to-noise ratio (SNR) is lowered.
(3) Maximal Ratio Combining Method.
The maximal ratio combining method controls the sizes and independent phases of the respective received signals, and then combines these signals. More specifically, the strengths of the signals received through the antennas are detected, and then the strong signals are amplified and the weak signals attenuated. The signals are then all combined.
FIG. 1 is a view showing an example of an apparatus which performs a related-art selection diversity method. This apparatus comprises an antenna switching unit 3 for selecting one of antennas 1 and 2, a receiver 4 for recovering the received signal passed through the antenna switching unit and for outputting instant level information of the received signal, and a reference level changer 5 for outputting an average value of received signals output from the receiver as an adaptive reference level. The apparatus also includes a level comparator 6 for setting an upper reference level and a lower reference level from the adaptive reference level output from the reference level changer, comparing the instant level of the received signal output from the receiver to the upper reference level and to the lower reference, and for outputting a driving signal according to the comparing result. Finally, a switch driving unit 7 controls switching of the antenna switching unit based on the driving signal output from the level comparator.
In operation, the receiver 4 recovers the signal passed through the antenna switching unit and outputs the instant level information of the received signal. The reference level changer 5 calculates the an average value of the received signals based on the instant level information and outputs the average value as an adaptive reference level. The level comparator 6 sets an upper reference level and a lower reference level from the adaptive reference level output from the reference level changer, compares the instant level of the received signal output from the receiver 4 to the upper and lower reference levels, and outputs a driving signal to the switch driving unit based on a result of the comparison. The switch driving unit controls the switching operation of the antenna switching unit using the driving signal. When the strength of a signal received through one of the antennas is smaller than the reference level, the switch 3 switches to the other antenna for receiving a stronger signal.
FIG. 2 shows an example of an apparatus which performs a related-art in-phase combining diversity method. This apparatus comprises a phase selecting switch 12 and a phase shifter 13, a combiner 14, a tuner 15, a first amplifier 16, a local oscillator 17, a mixer 18, an intermediate amplifier 19, a demodulator 20, a receiving electric field strength detector 21, a controlling unit 22, and a logic circuit 23.
The phase selecting switch and the phase shifter perform a phase shift for a first signal received through one antenna (first antenna 11a) with a predetermined angle interval. The combiner 14 combines the phase-shifted first signal and a second signal received through a second antenna 11b. Tuner 15 selects the desired signal from the combined signal and the first amplifier 16 amplifies the signal output from the tuner. The local oscillator generates a local oscillating frequency, and the mixer outputs the amplified signal as an intermediate frequency signal using the local oscillating frequency. The intermediate amplifier 19 amplifies the intermediate frequency signal, and the demodulator demodulates the amplified intermediate frequency signal. The receiving electric field strength detector detects the receiving level of the amplified intermediate frequency signal, and the controlling unit compares the detected receiving level to a predetermined reference level to output a resulting value. Finally, the logic circuit outputs a switch control signal to the phase selecting switch based on the resulting value.
In operation, the signal received from the first antenna 11a is phase-shifted at one of 0°, 90°, 180° and 270° angles through the phase selecting switch and the phase shifter. The phase-shifted signal is then in-phase combined with the signal received from the second antenna 11b in the combiner. The combined signal is then converted into an intermediate frequency signal by passing through the tuner, the first amplifier, the mixer, and the intermediate amplifier. The demodulator demodulates the intermediate frequency signal, and then receiving electric field strength detector detects the receiving level of the intermediate frequency signal.
This diversity apparatus shifts the phase regularly so that the largest receiving level can be detected by the receiving electric field strength detector. That is, the controlling unit 22 compares the receiving level detected by the receiving electric field strength detector 21 to the reference level, and controls the phase selecting switch to maintain the present status when the receiving level is larger than the reference level. However, if the receiving level is not larger than the reference level, the controlling unit switches the phase selecting switch to a contact point which corresponds to a next phase angle. The phase-shift operation of the received signal is performed regularly in order to obtain the receiving signal level of larger strength.
FIGS. 3 and 4 show examples of apparatuses which perform related-art maximal ratio combining diversity methods. These apparatuses time-delay signals received through diversity antennas 32 and 52 using delaying devices 42 and 62, and then the signals are respectively combined with signals received through main antennas 30 and 50 using combiners 44 and 64.
To perform this function, the apparatus uses the characteristics of a CDMA communication system which expands data using one or more PN codes. That is, if the delayed time of the received signal on diversity antennas 32 and 52 generated by the delaying devices larger than one chip period of the PN code, the delayed received signal is uncorrelated with the received signal on the main antennas 30 and 50. Therefore, the received signal of the main antennas and the received signal of the diversity antennas combined in the combiners can be divided again by a rake receiver built in a DMA modem and maximal ratio combined. The diversity apparatus shown in FIG. 3 and the diversity apparatus shown in FIG. 4 have the same diversity receiving operation principles, except that diversity antenna 32 shown in FIG. 3 can only receive while diversity antenna 52 shown in FIG. 4 is able to both receive and transmit.
The diversity apparatus which performs the selection diversity method shown in FIG. 1 is able to achieve better received signal quality than a single-antenna system in a fading environment in which the strength of the received signal is changed according to the lapse of time. However, this apparatus is not able to improve received signal quality in a static environment in which the strength of the received signal is not changed according to a time lapse.
The diversity apparatus which performs the in-phase combining diversity method shown in FIG. 2 has a problem, in that the SNR of the combined signal output to the combiner is lowered which the strengths of the signals received through antennas 11a and 11b are different.
Also, phase shifter 13 and combiner 14 are located on a front end of the first amplifier 16. As a result, signal loss in the signal generated in the phase shifter and combiner increases an entire noise figure of the diversity apparatus. This reduces the receiving sensitivity, and lowers any improvement in the received signal quality that can be realized.
In addition, only the received signal of the first antenna 11a is phase-shifted. Therefore, imbalance between the average electric power of the first antenna and the average electric power of the second antenna is caused, which lowers the diversity receiving function.
The diversity apparatuses which perform the maximal ratio combining diversity method shown in FIGS. 3 and 4 use a delaying device which delays the received signals for at least one chip time of the PN code, or longer. When a PN code having 1 MHz chip rate is used, the delaying device delays at least 1 μs or longer (5 μs˜10 μs is desirable in the diversity apparatus shown in FIGS. 3 and 4). A circuit device for delaying an analog signal for more than 1 μs generally has large volume and therefore it is difficult to apply the device to a terminal in a mobile communication system (mobile station). Also, the circuit device generates a significant amount of loss in signal electric power. Consequently, the receiving sensitivity is lowered and imbalance between the average electric power of receiving signals is generated, which lowers the receiving function.
The general diversity apparatus controls diversity based on received signal strength indication (RSSI) information. However, since many base stations transmit signals with different offsets simultaneously in a same frequency band such as in a CDMA system, interference between signals has a profound adverse affects on signal quality. That is, in a CDMA system signal quality can differ as a result of interference from other base stations using a same RSSI value. Therefore, it is difficult to control diversity using the general diversity apparatus.
High data rate (FDR) method is a wireless packet data transmission technology based on CDMA which is able to transmit data in mega-units. If the general diversity apparatus were applied to an HDR mobile communication system, accuracy in signal quality measuring would be substantially lowered. Therefore, it is difficult to control the diversity accurately with related-art methods.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.